Inverter circuits are used to convert DC to AC power. For example, in a variable speed, constant frequency (VSCF) electrical system as may be used in an aircraft, variable frequency AC power is rectified and the DC converted to fixed frequency AC. Typically, semiconductor switches, as power transistors, are used in the inverter circuit.
It is desirable to monitor the switch current of an inverter to provide information for control functions. It is known to use a current transformer for sensing the switch current. See, for example, Mehl U.S. Pat. No. 4,609,832, FIG. 2, and Chetty et al. U.S. Pat. No. 4,542,440, FIG. 4. Mehl senses switch current to control an incremental base drive for a transistor switch of an inverter. Chetty has separate current transformers for each switch. It is also known to use a single current transformer for both inverter switches as shown in Hunter U.S. Pat. No. 4,002,963. This reduces the number of parts and circuit connections.
A problem sometimes encountered in an inverter circuit is a "shoot-through" condition in which both switches conduct at the same time. The switches form a short circuit across the DC power source and the current is excessive, often destroying the switches. The inductance of the primary windings of the current transformers limits the transient current and can avoid switch damage. In Hunter, however, the two primary windings are oppositely poled, the flux in the current transformer cancels and there is insufficient inductance to limit the current.